1. Field of the Invention
The present invention relates to an olefin type thermoplastic elastomer, and a sheet molded product and a laminate using this olefin type thermoplastic elastomer. Particularly, it relates to an olefin type thermoplastic elastomer, with which bleeding of a softening agent is less likely to take place, which is excellent in oil resistance, flexibility, mechanical strength, rubber elasticity and moldability, and an extruded product of which has a smooth surface, few granular structures and is excellent in outer appearance.
2. Discussion of Background
In recent years, from the viewpoint of rationalization of processes, recycling, etc., attention has been paid to thermoplastic elastomers such as styrene type, olefin type, ester type, amide type and urethane type materials which have moldability similar to that of thermoplastic resins, and which do not require a vulcanization step. These thermoplastic elastomers are widely used in components for automobiles, components for home electric appliances, components for medical instruments, electric wires, general merchandise, etc. Particularly, a partially crosslinked olefin type thermoplastic elastomer obtained by crosslinking an ethylene-α-olefin type copolymer rubber by heat treating a polypropylene type resin and an ethylene-α-olefin type copolymer rubber in the presence of an organic peroxide, has become well known.
However, conventional olefin type thermoplastic elastomers are poor with regard to flexibility, mechanical strength, rubber elasticity, etc., when compared with vulcanized rubber, and thus their use has been limited. In order to improve such properties, it has been attempted to impart flexibility by adding a mineral oil type softening agent or an organic peroxide non-crosslinked hydrocarbon rubber-like substance to the elastomer. Increasing the degree of crosslinking can improve rubber elasticity. However, even though crosslinking improves rubber elasticity, other changes such as a decrease in flexibility, decrease in mechanical strength or bleeding of the softening agent on the surface of the composition, may take place, and thus excellent physical properties are difficult to obtain.
In order to overcome such problems, a composition obtained by partially crosslinking an olefin type plastic and an oil-extended olefin type copolymer rubber obtained by adding a mineral oil type softening agent to a solution containing an olefin type copolymer rubber having a 100° C. Mooney viscosity of from 170 to 350, followed by removal of the solvent, has been proposed (Japanese Patent No. 2140072). However, the improvement in mechanical strength of this composition is inadequate. Further, when this composition is subjected to extrusion, the resulting molded product has a rough surface with a large number of small protrusions (called granular structures) which makes it difficult to obtain a molded product having a smooth surface.
In general, an olefin type thermoplastic elastomer is difficult to mold into a complicated shape by extrusion, particularly by contour extrusion, and a molded product with excellent smoothness of the surface is difficult to obtain. On the other hand, outer appearance and surface smoothness are very important for a molded product in the form of a thin sheet. Whether a material can be used for an application depends upon the purpose of use. In some cases the high gel content in the olefin type thermoplastic elastomer may lead to the formation of granular structures or may cause surface roughening in some cases and may render a material unsuitable for a given application.
Olefin type thermoplastic elastomers form a morphology (dispersion state) comprising a crystalline polypropylene resin as a matrix and olefin type rubber particles as domains in the matrix. The physical properties and characteristics of the olefin type thermoplastic elastomer are greatly related to the degree of dispersion of the olefin type rubber particles. It has been known that the physical properties of olefin type thermoplastic elastomers improve when the particles of the olefin type rubber are microscopically dispersed fine particles of from 1 to 2 μm. Extrudability of the olefin type thermoplastic elastomer and the surface characteristics of the extruded product are also greatly related to the morphology of the olefin type thermoplastic elastomer.
Conventionally, a physical means has been employed as a method of finely dispersing the olefin type rubber particles for improving morphology. For example, a means to improve dispersibility of the rubber by using a high-shear process with a machine such as a high-speed twin screw extruder may, for example, be employed. The fine dispersibility of the rubber particles can be increased by optimizing the screw constitution of a high-speed twin screw extruder to increase L/D of the extruder. Further, it has been attempted to improve dispersibility by making the elastomer pass through the extruder twice.
However, even by finely dispersing the rubber particles by such a physical means, it has conventionally been difficult to obtain an olefin type thermoplastic elastomer capable of forming an extruded product having few granular structures, having a smooth surface and excellent outer appearance with favorable extrudability.
The present invention overcomes the deficiencies of the prior art olefin type thermoplastic elastomers.